In a known method, a negative data as well as a positive data are transmitted from a sensor to a control unit. In the above method, the positive data is a data to be transmitted, and the negative data is a reversed data of the positive data. In this method, by performing comparison between the positive data and the negative data on a bit basis, it becomes possible to determine whether or not the data is changed during transmission, that is, whether the transmitted data has an error.
In Patent Document 1, the sensor generates the positive data and the negative data with different data generation circuits. Thus the control unit can detect a data error resulting from failure of the data generation circuit.
Patent Document 1: JP-S60-93844A
In Patent Document 1, the positive data and the negative data are based on a same data which is set by a data setting circuit. The data which is set by the data setting circuit is converted from an electric signal outputted from a detector of the sensor. Therefore, when the detector of the sensor fails, the positive data and the negative data are generated based on an error-contained-data which is set by the data setting circuit. In this case, the control unit determines that the transmitted data is correct. Thus the data error resulting from failure of the detector of the sensor cannot be detected and the data transmitted to the control unit has low reliability.